I. Field of the Invention
The present invention relates to communication systems, and more particularly, to the enhanced conversion of wideband speech signals to narrowband speech signals.
II. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term “cellular” systems encompasses both cellular and personal communications services (PCS) frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems for data, etc. are promulgated by the Telecommunication Industry Association (TIA), the International Telecommunications Union (ITU), and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and fully incorporated herein by reference. An exemplary described system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in draft versions of IS-2000 and has been approved by the TIA. The cdma2000 proposal is compatible with IS-95 systems in many ways. Another CDMA standard is the W-CDMA standard, as embodied in 3rd Generation Partnership Project “3GPP”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.
In a traditional landline telephone system, the transmission medium and terminals are bandlimited to 4000 Hz. Speech is typically transmitted in a narrow range of 300 Hz to 3400 Hz, with control and signaling overhead carried outside this range. In view of the physical constraints of landline telephone systems, signal propagation within cellular telephone systems is implemented with these same narrow frequency constraints so that calls originating from a cellular subscriber unit can be transmitted to a landline unit. However, cellular telephone systems are capable of transmitting signals with wider frequency ranges, since the physical limitations requiring a narrow frequency range are not present within the cellular system. An exemplary standard for generating signals with a wider frequency range is promulgated in document G.722 ITU-T, entitled “7 kHz Audio-Coding within 64 kBits/s,” published in 1989.
In the transmission of speech signals, the perceptual quality of the acoustic waveform is of primary importance to users and service providers. If a wireless communication system transmits signals with a wideband frequency range of 50 Hz to 7000 Hz, a conversion problem arises when a wideband signal terminates within a narrowband environment that attenuates the high frequency components of the wideband signal. Hence, there is a present need in the art to be able to convert a wideband speech signal into a narrowband speech signal without the loss of acoustic quality.